Rebuilt Vortec 5.7 runs rough after 15 minutes

Hi all.
I just rebuilt my 5.7 vortec engine in a 1996 chevy suburban 1500 4x4. Had the block all worked over by a machine shop. Had them redo the heads (valve job etc) and redid the rings etc. New oil pump everything pretty much new. This has a new upper & lower intake and new injectors as well.

Problem: After first start up the suburban runs fine. Drive for 15 minutes or so and it acts like a plug wire is missing and tries to shake itself apart.

What I have done: I took it to oreillys and borrowed their code reader. It said that cylinder 8 was misfiring. I took it home and checked compression in all cylinders. All are ok. I then took off the valve cover and readjusted the valve for cylinder 8. I put it all back together and had the same problem again. This time cylinder #3.

I will be taking off the drivers side valve cover and checking valves on #3 and will retest.

My Questions: 1. Can anyone think of a code or reset procedure or anything like it that I would need to do after rebuilding this engine? Someone was telling me that the throttle needed to be relearned and that I had to turn the engine on twice while putting my finger in my ear to make that happen
2. I cant seem to get a straight answer on how to adjust the valves on this. What I have been doing is starting the engine, loosening the valve until it starts clacking. Then I tighten it until it just barely stops clacking. Then I take it 1/2 (one half) turn further. Is that correct?

Hey Jone, welcome to the GM truck club. I don't know about the "relearning of the ECU" Since your compression check was good on all cylinders, I don't think this is a valve adjustment issue. I would think either an electrical misfire, (plug or wire) or a fuel injector "lean misfire".
Did you change spark plugs in it when you did the rebuild? There have been many discussions on here about the vortecs only likeing ACs'

Thanks for the quick reply and the welcome.
I put new ac plugs/wires. This was a complete rebuild. New distributor etc. Replaced cam and knock sensors etc. The only thing really not changed was vacuum lines. I also need to look at this thing at night while running and see if there are any sparks/light shows.

THe lean misfire on the injector would probably be an airflow sensor or something wouldnt it? It is a new injector set at a whopping 350.00 bucks.

I'm pretty sure I need to double check the valve adjustments though. I think they might have been tightened down too much when first installed. I also need to recheck compression on each cylinder and write down the numbers. I think a couple of them were kind of low for a newly rebuilt engine (90 lbs or whatever). Will a valve adjustment impact the compression poundage that shows up during a compression test?

Thanks for the info on the o-rings. I'm not sure if I damaged them or not. How would I be able to tell? Pretty sure I got the lifetime warranty on the injectors now, so if they are bad i'll rip em out and take them back in. It isn't easy to get at the injectors under the upper intake manifold - alot of crap to take off. Is there a way to tell if I have a problem with the injectors without removing the upper intake?

As for the compression - the engine was cold when I tested it last time. Do you know (or have a procedure) what steps I should take to check the compression. Sit at idle for 30 minutes or ? It's hard to remove plugs and put a compression tester in there when it is hot.

Yes if the valves are too tight it will cause low compression & your's IS low. Gas won't even fire much below 95-100 PSI. If the valves aren't closing, or barely closing, the air/compression can leak out when the pistons are trying to compress it. Did you do your preliminary V adj according to a shop manual? Also for an accurate compression check you need to have the engine warmed up to operating temp. 195 - 200 degrees. This is where it runs 99% of the time & you need to know what it is under these running conditions. It could also be a little low if you haven't run it long enough to break in the rings, but not that low.

Thanks. Yep. My friend and I did the initial according to the manual but I know it wasn't right for cylinder 3. I think I just need to redo it.
I've been told to loosen them when it is warmed up until they are clacking then just tighten until it stops clacking and 1/2 turn more. Is that correct. Somewhere on this board i've read 3/4's turn more.
I will also redo the compression test and make better notes of exact readings per cylinder.

This was my first rebuild so i'm a little behind the curve.

Also, I had the cam worked on but I am using the same roller cam and lifters/rods etc but the rings are all new. I was mostly concerned that I had a ring that wasn't correct since I put them on myself. BUt if I have 90 compression then my rings are working just probably a valve issue dont you think?

Jone, Do you have the individual rocker arms that GM has used for eons with one adjusting nut in the middle of each rocker arm? If yes, then 1/2 or 3/4 turn is fine. Warm up the engine till it's hot,pull the valve cover off one side at a time, loosen each nut till the rocker clatters, slowly tighten it down till it stops clattering, then turn it 1/2 to 3/4 turn. If you choose 1/2 turn & a month from now, you notice the lifters rattleing for 5 or 10 seconds in the morning on cold starts, then turn them another 1/4 turn.

After your VA re-check your compression & post it here.

You had the cam worked on but you used the same one?? I don't understand that part..

Did you put the rings on the pistons yourself? Did you know there is a top of the rings & a bottom? They are tapered on the edge, & only work properly if they are installed correctly. This could cause a compression problem if they're upside down.

For checking to see if your injector O rings are leaking you can warm it up & spray some carb cleaner around the O rings. If they're leaking the rpms will pick up.

If you got your initial valve adj wrong on no#3 then you probably got the firing order out of sequence so you may have other valves too tight so do the V.A. first. If it still hasn't straightened out your running problem, then move on to spraying the O rings.

Sorry Stephan. Let me pull my head out. The shop I took the motor to worked over the crank (not the cam). They said the cam was fine and didn't need to be adjusted. He also said that would minimize the break in issues (using the old cam/lifters/rods etc..). Yep, the rocker arms are the ones with the single bolt in the middle.

Yep. I think I got the rings on properly. I compared with the old ones and the instructions were pretty clear. I was concerned that I didn't have the break in the rings opposite each other (if that is the right term). I'm really a novice, but the guy I was working with did most of the work and he had done it before.

I'm not sure how to spray those o-rings. They are under the upper intake manifold on the vortec motor (unless i'm confused again and not clear on where I should do this) and I have to remove the upper intake manifold to get at the injector assembly. Can I take the upper intake off and still run the motor with all the vacuums and stuff off?

No you can't run it with the intake off. I thought you had access to spray the O rings from what you said earlier. I would pass on the O rings till last. If you guys were carefull they should be alright.
I'm hoping the valve adjustment will cure this. I'm glad you got the rings on right side up cuz that would have been a big job having to go back in & pull the pistons.
X2 on what the shop told you about breaking in the cam at the same time you're breaking in a new engine. You have to run them at 2,000-2500 RPMs for 30 minutes & sometimes that can raise h*ll with the new rings & bearings.
I'm still wondering about this "relearning for the ECU". There will be some ECU experts on here tomorrow. I think you should start another thread tomorrow & title it Relearning the ECU, or Reprogramming the ECU. A title like that will get the experts to click on your question & they will be able to tell you exactly what it means & whether you need to do it. They'll also be able to tell you how to do it. I have seen people talk about that on here but I think that was only if the ECU was replaced, not because the engine was taken out, but I don't remember for sure.